White Dwarf Stars: Cinders in Space
After a mid-sized star ejects its outer layers as a planetary nebula and runs out of fuel, what remains is a blazing-hot mass of carbon and densely-packed electrons called a “white dwarf”. Some 97% of all stars, including the Sun, will end their lives as a white dwarf. These are dim objects, but a few are bright enough for you to see with a backyard telescope.
The Basics
• A white dwarf is essentially a stellar cinder; it produces no heat of its own, but radiates away residual heat from the star’s core over many billions of years. White dwarfs have a temperature of 5,000-40,000 degrees, but they have a low surface brightness.
• Typical white dwarfs have a mass near that of the Sun, but a radius of the Earth. So a white dwarf is very dense: 1 cubic centimeter has a mass of 1,000 kg!
• A thin skin of gaseous hydrogen or helium surrounds each white dwarf. Light passing through this hot gas allows astronomers to figure out the temperature, composition, and even the magnetic field strength of a white dwarf.
A Deeper Look
• The electrons in a white dwarf are stripped from the carbon atoms and tightly squeezed together. When this happens, they exhibit a strange quantum-mechanical effect called “degeneracy pressure” that holds the star up against the crushing pull of its own gravity.
• But there’s a limit to how much the electron pressure can withstand. If the mass of the white dwarf exceeds about 1.4x the mass of our Sun, the star will continue to collapse further into an even stranger body called a “neutron star”. This upper mass limit is called the “Chandrasekhar Limit” after the brilliant Indian physicist who discovered it when he was only 21 years old.
• Eventually, every white dwarf will radiate away its residual energy and become a “black dwarf”. This takes billions of years, however, and the universe is not yet old enough to contain any black dwarf stars.
An image of Sirius A and B (from McDonald Observatory). The light spikes are instrumental artifacts.
Good To Know
Sirius, the “Dog Star”, has a companion star called Sirius B. Also known as the “Pup”, Sirius B is the brightest white dwarf in the sky. The Pup was first spotted by Alvan Clark in 1862.
Personal View
Because Sirius B is so close to the dazzling Sirius A, it’s extremely hard to see in normal conditions, even though it’s fairly bright at magnitude 8.6. Here’s one trick to see Sirius B that’s worked for me… look for the Pup using high magnification at twilight before the glare of Sirius A overwhelms its faint companion. The stars lie about 8″ apart right now, and will get as far as 11″ apart by 2025. That’s far enough apart for most telescopes to resolve. But bright Sirius overwhelms the Pup, so it’s still tricky to see. Try to see the Pup for yourself when Sirius becomes visible again in the early morning sky in August or September.
